I just got a used Macbook Air with 128 GB drive. It had Big Sur, but after reset it's back at Mojave. So this OS must have been saved on the disk [Edit: no, it was probably downloaded]. Will I save storage space if I create a bootable USB-stick with Big Sur, format the disk and perform a clean install, instead of upgrading?

  • The system volume does not have 'spare' old OSes on the disk ready to install. As Tetsujin points out: it was either on the Recovery Partition, or it was downloaded at the time of installation using one of the Internet Recovery methods.
    – benwiggy
    Jun 7, 2021 at 10:13
  • Aha @benwiggy. But if it were on a recovery partition, that partition requires space on the disk, and could be removed in a clean install. Anyway it was probably downloaded. I don't see a recovery partition in disk utility. Jun 7, 2021 at 11:41
  • The recovery partition is not removed on a clean install - one is even added if it is not present, or adjusted to be the same as the newly-installed OS if it is. The Disk Utility app doesn't show all partitions. Quick composite image showing the difference between The Disk Utility app & Terminal's equivalent… i.stack.imgur.com/FixX3.png
    – Tetsujin
    Jun 7, 2021 at 12:16

1 Answer 1


I doubt the end result would make enough difference to be worth the effort.

You'd increase usage during the upgrade process as opposed to clean & install, but by the time it's finished, both methods will end up with the same result.

The other advantage of updating would be if you'd already set up your iCloud login etc.

You may, btw, have got Mojave because of the recovery method used to clean install -

  • Cmd ⌘ R
    would reinstall the latest macOS that was installed on your Mac, without upgrading to a later version
  • Opt ⌥ Cmd ⌘ R
    would upgrade to the latest macOS that is compatible with your Mac
  • Shift ⇧ Opt ⌥ Cmd ⌘ R
    would reinstall the macOS that came with your Mac, or the version nearest to it that is still available.
  • I concur: there's just extra work, and no real space saving.
    – benwiggy
    Jun 7, 2021 at 10:11

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